1. Field of the invention
The invention concerns the supply of power to and the control of a laser for a fiber optic link between a subscriber and a telecommunications central office, the data to be transmitted having a high data signalling rate, of several tens or possibly hundreds of Mbit/s, and modulating the power supply current to a laser diode emitting a light beam transmitted by optical fiber. Hereinafter the laser diode will be referred to as a laser.
2. Description of the Prior art
Existing laser power supply devices are implemented in discrete components and integrated circuits.
Discrete component devices have the following disadvantages:
their poor accuracy due to the large variations in discrete component parameters with temperature, these variations being difficult to compensate,
they take up more room on printed circuit boards,
large-scale manufacture is more costly using them,
implementation problems: the high data signalling rates require short connections and therefore the use of hybrid components, further increasing costs.
Like discrete component devices, integrated circuit devices which already exist do not allow the laser power supply current to be adjusted or cut off if there is no data to be transmitted in order to increase the service life, or the use of two switchable data sources. In standard telecommunication practice a fiber optic link is connected at each end to a terminal module comprising a receive circuit and a transmit circuit each connected to an optical fiber if the link comprises a transmit fiber and a receive fiber or to a single optical fiber if the link comprises only one fiber for both transmission directions. To carry out from the telecommunication central office tests on the link connecting the central office and a terminal module, a test device located at the central office sends test signals to the terminal module and should receive in return the signals transmitted, so that it can check them. This requires loopback between the receive circuit and the transmit circuit in the terminal module. The laser power supply device is part of the transmit circuit and must therefore in this case receive the test signals from the receive circuit in order to retransmit them, &he receive circuit then constituting a data source, so to speak. What is more, in normal operation the power supply device receives data to be transmitted direct from a direct source connected to the terminal module. It must therefore be possible for the laser power supply device to be connected to one or other of these two sources. Also, loopback may also be used when a subscriber receives data of any kind--video, telephone, computer data, etc--for checking by the subscriber or the station which transmitted the data.
An object of the invention is to alleviate the disadvantages of existing devices, to provide for the use of two switchable data sources and to enable the laser power supply current to be adjusted and cut off.